Ceramic Pressure Device for Preparation of Known Gas Concentrations at the 100 Parts-per-Million Level Herman D. Axelrod, Ronald J. Teck, and James P. Lodge, Jr. National Center for Atmospheric Research, Boulder, Colo. 80302 Raymond H. Allen R . H. Allen Co., Boulder, Colo. 80302
RECENTEMPHASIS on the preparation of known diluted gas streams for the calibration of equipment and the evaluation of analytical techniques has been in the area of 1 ppm (parts per million) and below. O’Keeffe and Ortman ( I ) have described a Teflon (Du Pont) permeation tube for the production of small quantities of gases. Axelrod et al. (2) have used a gas dosing device for similar purposes. Neither of these systems, however, is useful for concentrations in the 10 to 1000 ppm range. This area is of interest to those who wish to simulate stack sample concentrations. The common technique for the preparation of higher levels has been the use of a mechanically driven syringe to introduce large quantities of the test gas into the sample stream. A complete review of the syringe and other techniques has been presented by Lodge (3). An alternate procedure to injection by a syringe is to force gas through a finely drawn capillary from a high pressure tank. Drawbacks to this technique include changes in the capillary due to accumulation of particulate matter, and the capillary’s fragility. Below is described a device which can be used for the production of gases in the 10 to 1000 ppm range. Instead of the capillary, a porous ceramic frit is faced to heavy-wall glass tubing and attached to a high pressure gas cylinder containing the test gas. The pressure behind the frit can be adjusted by a regulator to provide precise flows of test gas. The surface of the frit is swept with a diluting gas, such as nitrogen or air, and the total output can then be passed through a manifold for sampling. This enables the experimenter to produce a variety of gas concentrations in a dynamic system and thus to simulate stack effluences. Figure 1 shows a model of the dilution system. The ceramic frit can be varied in length to give various flows for a given regulator pressure. The frit material is made of Mullite or Aluminate ceramic, available from the Coors Porcelain Co., Golden, Colo. The device is easy to construct, and, if made with heavy-wall glass, can withstand pressures greater than 20 psig. Because the device is constructed from glass, there is always a danger of explosion from the pressurization of the test gas. Therefore, the device should be placed in a hood and wrapped with tape or metal gauze or kept behind a safety shield. An alternate precaution would be to place a pressure safety valve in the system with appropriate controls for venting noxious gases. It is, of course, obvious that each device should be pressure tested prior to use. The diluter is far less expensive than the typical mechan(1) A. E. OKeeffe and G. C . Ortman, ANAL.CHEM., 38,760 (1966). (2) H. D. Axelrod, J. B. Pate, W. R. Barchet, and J. P. Lodge, Jr. A m o s . Enuiron., 4,209 (1970). (3) J. P. Lodge, Jr., “Air Pollution,” Vol. 11, 2nd ed., A. C . Stern, Ed., Academic Press, New York, N. Y., 1968,p 465.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
Sample Gas
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Figure 1. Ceramic gas dilution device
s 60
E 1 E
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Figure 2. Typical calibration curve for ceramic gas dilution device ically driven syringe system, and is more reliable than the capillary because the gas is dispersed through many holes in the porous material rather than through one small capillary hole which can become damaged or altered. The calibration of the flow of test gas through the frit in relation to the regulator pressure can be accomplished through the use of a wet test meter or a soap bubble flow meter. Either of these will give accurate results. Figure 2 shows a typical calibration curve for a hollow Mullite ceramic cylinder section 3 mm long, 6 mm in diameter, and 2-mm-wall thickness. By varying the pressure behind the frit and the flow rate of the diluting gas, the experimenter can produce various concentrations of test gas. Similarly, altering the diameter and thickness of the ceramic frit will also change the flow rate. For an accurate system, a flowmeter would be needed for the diluting gas, but once the frit had been calibrated, further calibration would not be needed. RECEIVED for review November 9,1970. Accepted December 10, 1970. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
